Ive got a little trick!
this has happened to me so many times, I've stopped counting. I have no idea how I get so lucky to have them come back because by the responses I've read, not everyone is as lucky as I am..
This is what has worked for me:
--Do everything everyone has already said with pushing volume UP, DOWN, then hold volume up + lock button to turn the phone off on some kind of "system reset"
--Once the phone is back on, make sure you are properly logged into your iCloud, go to settings, your name, iCloud, and make sure Maps is toggled on to be using iCloud.
--Delete the Apple Maps and reinstall in from the App Store.
**This did the trick the FIRST time.
HOWEVER. Not always. What I recommend MORE THAN ANYTHING is to share the guide (or guides) with yourself via email so that when this happens again, you can click the link from the email and in my experience, maps then gets triggered and gets its **** together.
**To do so, when you are on maps and you scroll down to see your guide, slide to the left and use the share button to either email or copy (and paste in an email).
If you are able to get your guides back with the first few steps-- hallelujah!
But if you have to restart your guide, keep this in mind for when you add more to the guide with time, since I am unsure of how the guide will populate after clicking the emailed link. It may only bring back the guides that were saved at the time of sharing them with yourself.
I try to remember to email it to myself once a month or two since this happens to me too frequently and I use them as I travel for work.
Hope This Helps-- Best of Luck!!!!